suppose: 1) p is random point on line PQ, 2) the connection line of two ... four triangles in the graph which drawed based on the above suppose, they are p b1 o1, p a o1 p b2 o2, p a o2 inorder to prove PQ...

'Supposed to' usually means that it would be the correct thing to do. Saying...should be, or confirming that we are in the right queue. I'm supposed to look for good flowers, means that's what I should do...